1. Field of the Invention
The invention relates to an X-ray examination apparatus which includes an X-ray detector for deriving an optical image from an X-ray image, an image pick-up apparatus for picking up the optical image and including a controllable amplifier for producing an image signal and an exposure control system for adjusting the controllable amplifier. The invention also relates to a method of controlling a controllable amplifier of an image pick-up apparatus for picking up an image, in which the controllable amplifier produces an image signal representing image information in the image.
2. Description of the Related Art
An X-ray examination apparatus of this kind is known from U.S. Pat. No. 4,703,496. A method of the kind set forth is apparent from the operation of the X-ray examination apparatus described in the cited patent.
The known X-ray examination apparatus includes an image intensifier pick-up chain with an X-ray image intensifier and an image pick-up apparatus in the form of a video camera. The X-ray image intensifier converts an X-ray image into an optical image and the video camera derives the image signal from the optical image. The video camera includes a controllable amplifier. The exposure control system includes a photosensor for measuring a brightness value of the optical image. The photosensor signal represents the mean brightness in the optical image. Furthermore, the photosensor signal anticipates the desired gain factor of the controllable amplifier. This means that the amplifier is adjusted on the basis of the mean brightness of the optical image, and hence that the amplifier is adjusted to bring the signal level of the image signal within a desired range. The image signal is applied to a monitor for displaying the image information of the X-ray image.
The transfer characteristic between the optical image on the exit window of the X-ray image intensifier and the output of the video camera is required for the adjustment of the amplifier of the known X-ray examination apparatus on the basis of the mean brightness of the optical image. The transfer characteristic describes the relation between the signal level of the image signal and the brightness of the optical image. Because the transfer characteristic is complex, notably non-linear, complex and time-consuming calculations are required so as to achieve accurate adjustment of the controllable amplifier. Consequently, either much time is lost before the controllable amplifier has been suitably adjusted or the amplifier is inaccurately adjusted. In the latter case image information of the X-ray image will be lost, or not clearly reproduced. If too much time is required for accurate adjustment of the controllable amplifier, image information of a fast succession of X-ray images cannot be reproduced without giving rise to disturbances in the image signal.